Between 30 and 50 percent of the world's population is believed to be infected with the Toxoplasma gondii parasite, but very few people have symptoms because the immune system usually keeps the parasite from causing illness. T. gondii is also found in animals, particularly domestic cats.

People usually contract T. gondii in two ways -- by eating the undercooked flesh of infected animals or through contact with cat feces.

The parasite causes a disease called toxoplasmosis. Most people experience mild flu-like symptoms before it goes into a chronic and dormant phase. But the disease can be fatal in people with weakened immune systems and in fetuses, which can be infected through the mother. This is why women are advised to avoid contact with cat litter boxes while pregnant.

Some studies have shown that mental health disorders such as schizophrenia, depression and anxiety are more common in people with toxoplasmosis, while others have suggested that the disease can influence a person's levels of aggression, extroversion and risk-taking, the researchers from the Karolinska Institute in Sweden pointed out.

In this new study, a laboratory experiment conducted by the researchers found that human dendritic cells -- which are an important part of the immune system -- started secreting the signaling chemical GABA after being infected with toxoplasma.

In another experiment, this time using mice, the scientists introduced the parasite to the brain, and then traced the movement of infected dendritic cells from the brain through the body where it spread and continued to affect the GABA system, according to the study published Dec. 6 in the journal PLoS Pathogens.

Among its roles, GABA inhibits feelings of fear and anxiety. Disturbances of the GABA system occur in people with depression, schizophrenia, bipolar diseases, anxiety syndrome and other mental health disorders.

"For toxoplasma to make cells in the immune defense secrete GABA was as surprising as it was unexpected, and is very clever of the parasite," study leader Antonio Barragan, a researcher at the Center for Infectious Medicine at Karolinska Institute and the Swedish Institute for Communicable Disease Control, said in a Karolinska news release. "It would now be worth studying the links that exist between toxoplasmosis, the GABA systems and major public health threats," he added.

"We believe that this knowledge may be important for the further understanding of complex interactions in some major public health issues, that modern science still hasn't been able to explain fully," Barragan suggested. "At the same time, it's important to emphasize that humans have lived with this parasite for many millennia, so today's carriers of toxoplasma need not be particularly worried."

While the study found an association between the immune system reaction to T. gondii and mental health issues, it did not prove a cause-and-effect relationship.

More information

The U.S. Centers for Disease Control and Prevention has more about toxoplasmosis.

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